You will be logged out automatically when you close your browser, unless you have ticked the 'remember me' box on the login page. If you tick the box our server will set a cookie on your computer, and that will log you in automatically each time you visit the site. The cookie is harmless, and no personal information is divulged to a third party by its use - it simply enables the site to recognise you each time you return.
If you use one of the widely available clean-up utilities -Like CCleaner - to clear your browsing history and cookie cache our cookie will be deleted, and you'll need to log-in again (and tick the 'remember me' box).
I hope that helps, let me know if you need any further information.